The present invention relates to light-load efficiency of power converters, and more particularly to smooth transition between heavy- and light-loads of power converters.
Light-load efficiency of power converters is becoming increasingly important for switch mode power supply (SMPS) powered devices. Improved light-load efficiency helps save energy and extend battery life of the SMPS powered devices. Conventional methods for improving the light-load efficiency of the power converters include: reducing the circulation energy and associated losses by running a power converter's circuit in a discontinuous conduction mode (DCM); reducing switching losses by reducing switching frequency at a light load; and shutting down unnecessary phase(s) at a light load of a multi-phase converter. When the load current is high, it is preferable to operate a power converter in a fixed frequency continuous conduction mode (CCM), which allows a fast transient response, higher efficiency, and a narrower-spread noise spectrum. The key to achieving the best overall performance is knowing when and how to transition between the two modes.
Currently existing solutions for improving light-load efficiency of power converters include a constant on-time with valley-mode control, a hysteresis control, and a duty cycle control with pulse skipping/burst mode at light load methods. In the constant on-time with valley-mode control method, the control switch is turned on with a fixed period of time. The off-time is determined by a time at which the output voltage reaches or hits the valley threshold of the comparator. The disadvantages of this control are: variable frequency control for all input and load condition; instead of the average output voltage, the minimum output voltage is regulated to reference; the control is affected by the comparator delay; reliance on a reasonably large equivalent series resistance (ESR) of the output capacitor to generate the output voltage ramp for off-time comparison; and a need of a high-gain, high-bandwidth, low-offset differential amplifier to sense the output voltage ramp.
The hysteresis control method is similar to the constant on-time control, having the same advantages and disadvantages. However, both on-time and off-time of the control switch are controlled by the hysteresis of the comparator.
In the duty cycle control with pulse skipping/burst mode at light load method, a fixed-frequency duty cycle control is used when the load current is high. At light load, the control switch is turned ON only when the output voltage is less than the threshold and within the hysteresis of a comparator. This results in the control switch turning on for a few consecutive cycles and then keeping silent until the output voltage drops below the threshold. This control is not desirable because a burst mode at light load can cause the switching noise spectrum spread to a wide range, imposing EMI issues.
It is clear that the existing control methods either have variable frequency control at all time or have an abrupt transition between fixed and variable frequency when transitioning between heavy load and light load condition. The ideal solution is to have constant-frequency duty cycle control at heavy load condition, while using constant on-time variable frequency control at light load condition through a smooth transition. As a result, both the efficiency and switching noise concerns can be taken care of.